The invention concerns a nuclear magnetic resonance (NMR) apparatus comprising a superconducting main field magnet coil system which generates a homogeneous magnetic field of at least 3T, and a gradient coil system which generates a gradient strength of at least 10 mTm−1, with a slew rate of at least 100 Tm−1s−1, wherein the main field magnet coil system is arranged in a cryostat with liquid helium having a refrigerator in the form of a pulse tube cooler or a Gifford-McMahon cooler, wherein an evaporation line is provided for helium that evaporates from the cryostat.
An apparatus of this type is disclosed in a publication by Lvovsky and Peter Jarvis [1].
Superconduction technology is used for generating strong magnetic fields in an economical fashion. Superconductors are energized at very low temperatures in order to be able to utilize the effect of resistance-free current conduction, and subsequently generate the corresponding magnetic field.
The superconductor is cooled by evaporation of liquid helium. For economic reasons, these evaporation rates must be minimized [2, 3, 5].
Accordingly, many NMR apparatus (cryostats) that are currently on the market have refrigerators [4].
In recent years, the technology has reached a state which permits re-liquifying of the helium that has evaporated in the quiescent state of the system (no switching of gradients). These systems are designated as so-called zero boil-off systems.
As is disclosed e.g. in [6, 7, 8], vibrations and eddy currents are generated in an NMR apparatus due to rapid switching of gradient fields, which then result in a considerably increased helium evaporation rate. This considerably shortens the refill cycles, thereby extending the downtimes of the system and reducing the effective useful life. The evaporating helium is usually simply introduced into the atmosphere or, in rare cases, also condensed and filled into gas bottles and moved from there to industrial scale plants where the gas is re-liquified. Helium is available on the earth only in limited amounts and for this reason, this resource must be handled economically. Introduction of the helium gas into the atmosphere does certainly not meet this objective. When the out-flowing gas is condensed and re-liquified in industrial scale plants, the above-mentioned objective is met. This is, however, accompanied by substantial logistic expense.
It is therefore the object of the invention to provide a high field NMR apparatus, which saves considerable amounts of evaporated helium, thereby considerably extending the liquid helium refill cycles which greatly disturb and obstruct normal work cycles and are disadvantageous for operation, in particular, in hospitals.